What is life and how can you tell if something is alive?

Many people have thought about the answers and most agree that there are seven characteristics or features of living things that make them different from non-living things.  To be alive, all must apply.

Standard biology textbooks often offer the following seven characteristics to define life:

Movement
RespirationSensitivity
GrowthReproductionExcretion
Nutrition

The respected biologist Edward Mayr elaborated on this in his book, The Growth of Biological Thought (1982).  He suggested the following eight points necessary for consideration in recognizing living systems and entities.  In simplified form his points were as follows:

• Living things are complex and can adapt in an organized way.
• The chemistry of life is made up from a unique set of large molecules.
• Each living organism is individual, and there is much variability between groups.
• Living things evolve in a non-predetermined way, but a record of their evolution is maintained in their genetic code.
• Living things are classified by their genetic evolution.
• All living things have evolved by natural selection.
• The processes of life cannot be easily predicted.
• Living things are best recognized by qualitative rather than quantitative characteristics.

In the light of recent discoveries, the very different environmental conditions found in space as compared with Earth, it is necessary to broaden and change some of the definitions.

Life must be or have:

...an organized pattern: STRUCTURE

...self-replicating, store and pass on instructions for same: REPRODUCTION

...the ability to convert energy: METABOLISM

...the ability to change structure: GROWTH

...the ability to change into new and different entities: EVOLUTION

...able to interact with surroundings: IRRITABILITY

...a certain robustness to changes in the environment: RESILIENCE

Crystals have some of these abilities, and fire others, but neither could be considered as living. Robots can be built to have yet more of the features of life, but they cannot grow physically, evolve or reproduce, so fail the life test on three counts.